The present invention relates to apparatus, systems and methods for fabricating a semiconductor device, and more particularly to, masking in a semiconductor fabricating process.
Semiconductors have become highly integrated and, as such, the component dimensions have become increasingly minute. Thus, there may be increased demand for mask pattern resolution to accommodate pattern dimensions that may be less than the wavelength of light used in an exposure apparatus.
Optical proximity correction (OPC) technology may be used to correct the shape of a mask pattern that may experience deformation caused by an optical proximity effect for a pattern having a shorter line width than the wavelength of light. Examples of OPC technology include, for example, model-based OPC and rule-based OPC. Model-based OPC can be more easily applied to various layouts than in rule-based OPC.
A current sub-50 nm device process may need a pattern scale of a mask of about 4 times, i.e. below 200 nm. However, a mask of this size may have a narrow structure, which may not be easily penetrated by an ArF laser used as a light source. As a result, internal scattering, mask induced polarization, and reflection loss due to pellicle may cause a 3-dimensional (3-D) mask effect.
Rigorous simulations may be performed to compensate for the 3-D mask effect, such as finite difference time domain (FDTD) analysis, rigorous coupled wave analysis (RCWA), and time-domain electromagnetic massively parallel evaluation of scattering from topography (TEMPEST) as a kind of FDTD. However, the rigorous simulations may be difficult to apply when OPC is performed over a large area. Further, such rigorous simulations may not give significantly better results than conventional simulations.
Reference is made to FIG. 1, which schematically illustrates a typical scanner system that may be used in photolithography. The scanner system includes a light source 10, an illumination lens 20, a mask 30, and a projection lens 40, among other components. An illumination pupil 22 may be formed on the illumination lens 20, and an imaging pupil 42 may be formed on the projection lens 40 to correct a pupil surface. A wafer 50, on which a pattern is to be formed, may include a resist layer 54 coated onto a silicon substrate 52. The wafer 50 may be placed under the scanner system and light may be irradiated onto the resist layer 54. Here, the mask 30 may include a light blocking layer 32 formed of chromium (Cr) and a light transmitting layer 34 formed of quartz. The silicon substrate 52 may be a pure silicon substrate or a silicon substrate on which multiple material layers have been formed.
A conventional mask may approximate a thin mask and OPC may be performed without considering the mask's thickness. However, the accuracy of a thin mask approximation may be lower when the feature size or pattern size of the mask approaches the wavelength of the light source, e.g. an ArF wavelength. In other words, in a current thin mask approximation method, mask Features are effective at above 2.5× the light source wavelength. Three-dimensional (3-D) mask effects, however, may occur below that size. In this regard it may be difficult to adopt the thin mask approximation method.
Reference is made to FIG. 2A, which is a cross-sectional view of a portion of a mask 30. The mask 30 may include a light transmitting layer 34 formed of glass and a light blocking layer 32 formed of chromium (Cr) underneath the light transmitting layer 34. Patterns may be formed in the light blocking a layer 32 to form openings of width W. Although one opening is illustrated by way of example, a mask 30 may include multiple openings corresponding to a pattern. Such an opening may generally have a width of about four times the pitch or line width of a wafer pattern.
Reference is now made to FIG. 2B, which is a graph of percent difference of a critical dimension (CD) of a wafer as a function of the width of an opening of a light blocking layer 32, as illustrated in FIG. 2A. The x-axis denotes the line width of a wafer pattern, and the y-axis denotes the percent difference of the CD. By way of example, the light source has a wavelength of 193 nm, the numerical aperture (NA) is 0.75, and the coherence coefficient σ is 0.35. Reference character M denotes the size factor of the opening of the mask 30 with respect to the line width of the wafer. Here, the factor M is “4.” As illustrated, the difference of the CD, which may also be referred to as error, increases abruptly at line widths below 150 nm. This can be attributed to inaccuracies in OPC which does not account for a 3-D mask effect as described above.